Seismic data acquisition is one of the geophysical tools used to image and characterize the earth's subsurface geological features. The data being acquired can be used to define regional or local geological structures and provide some detailed geological descriptions, making seismic data acquisition a widely used tool in hydrocarbon exploration and exploitation.
Seismic data acquisition uses sound source waves either generated at the surface, such as seismic vibrators or weight drops, or generated at some depth, such as with dynamite. These sound waves can be divided into 3 major categories, primary, secondary (shear), and surface waves. Hydrocarbon exploration generally utilises either primary or secondary waves. After being generated, primary and secondary waves propagate into subsurface geological layers and at the geological layers boundaries, part of them reflected and the other portions continue penetrating deeper layers. In seismic exploration, it is the reflected seismic waves that are considered as the key signals. Refracted and direct arrival seismic waves may to some degree be considered as signals whereas other types such as ground rolls and air waves are not. In land seismic data acquisitions sensors, such as geophones, are spread on the surface or down hole to receive the reflected waves as signals. The amplitudes of the reflected waves are used to scale the intensities of wave energy.
It is well known that amplitude varies with offset and incident/reflection angle. Seismic waves lose energy due to uncontrolled effects such as far offsets (distance from source), subsurface geology and fluids, and mechanical effects on reflection angles. This energy loss can be minimized through the claimed invention.
Many manufacturers suggest that in normal operation, P-wave geophones will function at an angle of 0-20° from vertical. In practice, under some conditions, P-wave geophones may be able to operate with reasonable accuracy at an angle up to 45° from vertical. To check how a live geophones is planted, a so called “Tilt Test” is run. The Tilt Test correlates known pulse inputs and outputs from a geophone string to a modelled pulse to produce a statistical ratio measuring the amount of deviation from vertical. In static conditions (passive source), extensive measurements were performed to check the effects of tilts on geophones' physical properties such as cutoff frequencies, damping, sensitivities, distortion, etc. These physical properties were observed unaffected when geophones are tiles at 0-30 degrees from vertical and this range can be extended to reach 40 degrees.
It has been found that if a geophone tilted at 40° from vertical and in a direction opposite the incoming signals, this will cause 70% loss in energy (R. Stewart et. al.). If the case is reversed (i.e. if the reflected signal angle is high (but less than critical angle) and the geophone is vertical), then energy losses are expected to be directly proportional to the reflection angles. When a seismic survey is designed, maximum offset is set to image the deepest desired reflector. For example, if the signal reflected at an angle of 45° from the deepest reflector, then geophones located at the maximum offset should be directed toward that angle in order to detect the maximum energy. From field experiments, however, if sensors planted at angles deviated at angles more than 30° but directed towards the reflected signal, it would give similar responses to those, which are planted vertically.
Recording signals reflected at different angles during seismic survey shooting is very difficult and impractical with the current system available in the market. These systems do not provide a way to overcome energy loss due to reflection angles. Consequently, the current systems may not generate a completely accurate seismic signal, which may in turn lead to misinterpreting the information.